The present invention generally relates to an image processing apparatus, and more particularly to an image processing apparatus in which an image is quantized through an error spreading process to output a bi-level or multilevel image to a printer. This apparatus is applicable to digital PPCs, scanners, and facsimila machines.
Conventionally, there is an image processing apparatus in which an input image is quantized through an error spreading filter. For example, Japanese Laid-Open Patent Publication No. 1-303869 discloses an image processing apparatus of this type. In this image processing apparatus, when an image is quantized through an error spreading filter, quantizing errors between the input data and the output data are always stored, and corresponding dots are output. If the input image has a certain level of density for a white background portion thereof, undesired dots or irregular edges appear in the output image for the white background portion.
In the conventional apparatus described above, a white background portion of the input image is discriminated by calculating an average of densities with respect to a target pixel and neighboring pixels surrounding the target pixel in the input image and by comparing the average of the densities with a suitably adjusted reference value. When it is detected that the target pixel is at the white background portion, the conventional apparatus described above does not output a dot with respect to that pixel so as to prevent the undesired dot to irregular edge from appearing in the output image.
However, in the above described apparatus, after the input image is quantized through the error spreading filter, undesired stripes or textures may appear in the output image for a highlight portion (or a white level portion) of a continuous halftone image such as a photograph, or irregular edges or noises may appear in the output image for an edge portion of a discontinuous density image such as a character. Hence, the picture quality is degraded when such an image is processed.
Moreover, several methods for image quantization through error spreading filters have been proposed in order to achieve image quantization with good picture quality of the output image. For example, a fixed threshold method utilizing "n-1" fixed threshold values to produce image data indicating one of "n" different density levels is one of the proposed methods. If the fixed threshold method mentioned above is used, the resolution of the output image for a discontinuous density image such as a character is satisfactory. However, an undesired pseudo outline may appear in the output image for a continuous halftone image such as a photograph, and an undesired moire may appear in the output image for a screened halftone image such as a dot photograph. Another one of the proposed methods is the multilevel dither method, and this method is useful because satisfactory gradation characteristics of the output image for a continuous halftone image can be produced. However, if the multilevel dither method mentioned above is used for a discontinuous density image being input, the resolution of the output image for the input discontinuous density image becomes poor, and a moire may appear at a halftone portion of a screened halftone image due to the interference between the image and the dither pattern being used.
As described above, in the cases of the conventional methods, it is difficult to make both the resolution and the gradation characteristics satisfactory after the input image has been quantized through the error spreading process, for an image selected from among a continuous halftone image, a screened halftone image, and a discontinuous density image.